niri-scratchpad: stop screencasts on stash; harden acceptance spawn wait

Stashing a window now stops an active screencast of it, rather than leaving the
consumer on a frozen last frame (a stashed window is never rendered). Covers
both stash paths - Mod+M and Mod+Shift+M's hide branch - by diffing which
windows left the layout across the action, so scratchpad_show's branch logic
isn't duplicated in the caller. Dynamic-target casts are retargeted to Nothing
rather than ended; that carve-out is documented inline.

Also widen the acceptance harness spawn wait to 30s: each test launches its own
nested niri, and a cold alacritty start occasionally exceeded the old 10s.
Deliberately not a retry, which could double-spawn and corrupt window counts.

117 unit tests and 8/8 live acceptance pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

niri-patches/0001-scratchpad.patch

@@ -168,29 +168,81 @@ index 38440c90..fe16901f 100644
              // involving laptop going to sleep and resuming.
              error!("toplevel missing from both unmapped_windows and layout");
 diff --git a/src/input/mod.rs b/src/input/mod.rs
-index f1ad4932..91bc8942 100644
+index f1ad4932..87b5ea7a 100644
 --- a/src/input/mod.rs
 +++ b/src/input/mod.rs
-@@ -1376,6 +1376,13 @@ impl State {
+@@ -54,6 +54,7 @@ use crate::ui::mru::{WindowMru, WindowMruUi};
+ use crate::ui::screenshot_ui::ScreenshotUi;
+ use crate::utils::spawning::{spawn, spawn_sh};
+ use crate::utils::{center, get_monotonic_time, CastSessionId, ResizeEdge};
++use crate::window::mapped::MappedId;
+ 
+ pub mod backend_ext;
+ pub mod move_grab;
+@@ -647,6 +648,38 @@ impl State {
+         }
+     }
+ 
++    /// Ids of all windows currently in the layout. Excludes the scratchpad stash,
++    /// which is not part of any workspace.
++    fn layout_window_ids(&self) -> HashSet<u64> {
++        self.niri
++            .layout
++            .windows()
++            .map(|(_, mapped)| {
++                // Annotated to pin the inherent Mapped::id() -> MappedId; the
++                // LayoutElement trait also has an id(), returning &Window.
++                let id: MappedId = mapped.id();
++                id.get()
++            })
++            .collect()
++    }
++
++    /// Stop screencasts of windows that were in the layout before an action and are
++    /// gone after it. A stashed window is never rendered, so its cast would otherwise
++    /// sit frozen on its last frame indefinitely; stopping is the chosen policy over
++    /// rendering stashed tiles.
++    ///
++    /// Note the carve-out inherited from `stop_casts_for_target`: a cast with a
++    /// *dynamic* target is not ended, it is retargeted to `CastTarget::Nothing`. So a
++    /// follow-focus cast survives a stash and goes blank rather than stopping. That is
++    /// shared behaviour with every other stop site, not specific to the scratchpad.
++    fn stop_casts_for_windows_gone(&mut self, before: HashSet<u64>) {
++        let after = self.layout_window_ids();
++        for id in before.difference(&after) {
++            self.niri
++                .stop_casts_for_target(CastTarget::Window { id: *id });
++        }
++    }
++
+     pub fn do_action(&mut self, action: Action, allow_when_locked: bool) {
+         if self.niri.is_locked() && !(allow_when_locked || allowed_when_locked(&action)) {
+             return;
+@@ -1376,6 +1409,15 @@ impl State {
                      }
                  }
              }
 +            Action::MoveWindowToScratchpad => {
 +                let focus = self.niri.layout.focus().map(|m| m.window.clone());
 +                if let Some(window) = focus {
++                    let before = self.layout_window_ids();
 +                    self.niri.layout.move_to_scratchpad(&window);
++                    self.stop_casts_for_windows_gone(before);
 +                    self.niri.queue_redraw_all();
 +                }
 +            }
              Action::MoveColumnToWorkspaceDown(focus) => {
                  self.niri.layout.move_column_to_workspace_down(focus);
                  self.maybe_warp_cursor_to_focus();
-@@ -2245,6 +2252,10 @@ impl State {
+@@ -2245,6 +2287,13 @@ impl State {
              Action::StopCast(session_id) => {
                  self.niri.stop_cast(CastSessionId::from(session_id));
              }
 +            Action::ScratchpadShow => {
++                // The hide branch of scratchpad_show also stashes a window.
++                let before = self.layout_window_ids();
 +                self.niri.layout.scratchpad_show();
++                self.stop_casts_for_windows_gone(before);
 +                self.niri.queue_redraw_all();
 +            }
              Action::ToggleOverview => {